During observation of such luminous effects, the sensitivity of most means of observation--e.g. the human eye--can be adjusted or it adjusts itself--within certain limits--to the intensity of the luminous effect to be observed. E.g. sensitivity of the human eye can automatically change or there is known such means of observation which can automatically change its sensitivity. This so called adaptation of the eye is physiologically limited and it often occurs such highly intensive luminous phenomena that cannot be observed without the serious damage of the means of observation, e.g. the human eye.
Such highly intensive luminous phenomena are especially frequent during heat power technological processes, i.e. welding or melting.
Naturally there are already several various means for the observation of luminous phenomena of especially high intensity compared to that of the ambient luminous effects.
Among such means with one or more light filters are in most common use at present, however, protecting means based on light polarization are ever more in general use. The unfavourable characteristics of simple masks and spectacles equipped with a light filter are well-known. Such an unfavourable characteristic is, e.g. that these equipment can usually be operated in two operational statuses, i.e. the protecting means is either inserted in or not. Consequently they are hardly suitable for the observation of luminous phenomena of variable intensity. There is another solution whereby reduction of light intensity to a certain degree can be achieved gradually, i.e. by inserting several light filters. The disadvantage of this means is that light intensity can only be regulated gradually, and it can be inserted by manual way or by a complicated mechanical solution.
Beyond these simple mechanical protecting means there are such ones whereas luminous intensity is reduced by optical grating or polarizers instead of light filters. The use of optical grating for such a purpose is not wide-spread due to its several unfavourable characteristics. The principle method of applying polarizers is that a liquid crystal is inserted between at least two polarizers angle of rotation of which depends on the applied voltage. Practical realization of this protecting means--e.g. in case of welding--is generally with two liquid crystal. Main characteristic of these well-known means is that the joint operation of liquid crystal is relatively slow, so a short overloading of the means of observation, e.g. of the human eye, is not excluded. Furthermore for the operation of most well-known means special voltage supply is required which harmfully effects operational safety, or the required operational safety can be granted only by special, expensive construction.
Furthermore, these well-known means can generally be operated in a binary way, meaning, that a luminous phenomenon of higher intensity than a preset value induces the means to reduce its light transmission to a certain value independently of the intensity of the luminous phenomenon inducing the reduction. In such a way the intensity of transmitted light is substantially changed.